A great many hydraulic devices have been developed over the years for tensioning endless roller chains and belts particularly in vehicular applications. Some prior art devices feature a pressurized fluid operated piston whose exposed end has been adapted to engage the chain or belt without unduly inhibiting its movement and is operative to displace the chain or belt transversely to tension it as the piston rod is caused to move towards the chain or belt in response to pressurized fluid being conveyed into the piston chamber. Other prior art devices include resilient biasing means such as a coiled spring in the piston chamber to either urge the piston rod towards the chain or belt in conjunction with the pressurized fluid or in some cases to urge the piston rod in an opposite direction to lessen the force of the pressurized fluid on the piston rod so that the roller chain or belt is not over tensioned. Generally, in such cases, the fluid selected is an incompressible fluid such as oil which acts as an incompressible medium within the fluid reservoir.
It has also been common practice to include a high pressure fluid chamber within the piston that receives an incompressible fluid such as oil from the fluid reservoir through an opening therebetween having a one-way check valve. Such devices operate on the principle employed for automotive fluid operated slack adjusters or valve lifters where, when no load is applied against the end of the device, the combination of a coiled spring and the fluid in the reservoir urge the piston against the valve cam or rod whilst the check valve enables the fluid to flow from the reservoir into the high pressure chamber until the fluid pressure within the high pressure chamber is the same as the fluid pressure in the reservoir at which point the check valve closes and seals the fluid in the high pressure chamber to act as incompressible fluid medium against which the cam or rod acts during its rotational cycle or stroke. The fluid pressure in the high pressure chamber will subsequently decrease due to the enlargement of the high pressure chamber arising by movement of the piston towards the rod or cam and/or due to leakage until it falls below the fluid pressure in the reservoir which again causes the check valve to open and enable the fluid to pass from the reservoir into the high pressure chamber to provide cyclic recharging of fluid in the high pressure chamber.
One of the problems associated with the use of such hydraulic tensioning devices is purging air from the piston chamber while it is being filled with the fluid and to enable entrapped air to escape from the piston chamber while it is operating, for air is compressible and may adversely affect the operation of the device.
Although it has been common practice to provide an air vent on the back side of a hydraulically operated piston to prevent the air from being compressed, little has been done in providing an air vent communicating directly with the pressurized fluid chamber itself.
An example of a hydraulic belt tensioner utilizing a singular air vent communicating with the center of a pressurized fluid chamber is disclosed in U.S. Pat. No. 4,276,038, the disclosure of which is incorporated herein by reference. In this case, the air vent enters the clearance between the piston and the surrounding bore and would necessarily be limited in its effectiveness according to orientation of the device.
An example of a hydraulic belt tensioner having only a singular fluid exhaust port at the exposed end of the piston which is plugged after the device is filled with oil is disclosed in U.S. Pat. No. 4,277,240, the disclosure of which is incorporated herein by reference. Here, however, no air vent enters the main pressurized fluid chamber and no provisions are made to vent air that may be entrapped in the pressurized fluid whilst the device is operating. A similar concept is disclosed in U.S. Pat. No. 4,283,181, the incorporation of which is included herein by reference and which also teaches the use of an air vent on the back side of the piston to prevent air from being compressed during the pressurized stroke of the piston.
An example of a closed hydraulic chain tensioning device featuring a hollow piston that enables air to escape while the piston is being initially filled with fluid and which is then plugged is disclosed in U.S. Pat. No. 4,504,251, the disclosure of which is incorporated herein by reference. Here, however, no provision is made to vent air that may be entrapped in the fluid during operation of the device.
An example of a tensioning device utilizing an opening to vent air on the backside of a cylinder is disclosed in U.S. Pat. No. 4,674,996, the disclosure of which is incorporated herein by reference. Again, however, no provision is made to vent entrapped air in the pressurized fluid while the device is operating.
The chain or belt tensioning device of the present invention not only enables entrapped air to be vented when it is being filled with the fluid and during its operation but also enables a broad orientation range of the device relative a horizontal plane without an appreciable amount of the fluid draining therefrom under gravity when not pressurized.